I used to work in a place that had a profoundly-deaf engineer. Apart from having a vibrating pager connected to the fire alarm, it didn't affect his work in any way. He could speak in the manner of a deaf person, initially hard to understand, but once you got used to it, it was possible to have a conversation with him.
One day, one of the other engineers rang up Mad Jason the IT guy, and did a very good impression of the deaf guy, Roger's exaggerated and slowed-down speech, saying he had a problem with his PC. Now, between putting the phone down and walking across to the office, Mad Jason had realised he'd just been apparently speaking on the phone to somebody who'd been totally deaf from birth. Most people at this point would realise that they'd been had...
Mad Jason storms up to Roger's desk, fuming and shouting "You bastard, I knew you weren't really deaf!"